If a steel blankwork containing more than 1% Si is hot rolled, generally the hot rolled sheet is recrystallized at the surface layer only, and the middle layer is composed of a rolled and non-recrystallized structure. If such a hot rolled sheet is cold rolled and annealed as it is, magnetic properties can not be provided, since a texture beneficial to the magnetic properties develops insufficiently. For securing the magnetic properties after the cold rolling and annealing, the hot rolled structure should be perfectly recrystallized. For example, Japanese Patent Application Laid Open Specifications No.68717/79 or No.97426/80, aiming at such objects, disclose annealings on the hot rolled sheet by a batch annealing or a continuous annealing after hot rolling and coiling.
In the annealing of the hot rolled sheet as such, if the recrystallization treatment is carried out on the hot rolled sheet, and if the annealing is done in an insufficient non-oxidizing atmosphere, the scales remaining on the surface thereof develop and grow thick, and internal oxidized layers grow in the steel surface layer so that a pickling ability after the treatment is markedly deteriorated. On the other hand, in spite of the non-oxidizing atmosphere, if the annealing is done in the atmosphere containing nitrogen, a nitriding reaction is accelerated in the steel surface layer, and it combines with Al in the steel and brings about precipitations of AlN in the steel surface layer. Therefore, AlN particles considerably lower ferrite grain growth in a final annealing. As a result, the steel surface layer is formed with regions of fine ferrite grains of about 20 .mu.m in thickness of about 100 .mu.m, which remarkably deteriorate properties of iron losses and magnetic properties at low magnetic fields.
In view of these circumstances, Japanese Patent Application Laid Open Specification No.35627/82 discloses an art of performing the pickling after the coiling at high temperature and subsequently a batch annealing. However, at coiling temperatures of higher than 700.degree. C., not only does the scale on the surface grow thick, but also an oxidation is caused in the ferrite grains, if the steel sheet contains more than 1 wt % Si. The oxidized layer in the ferrite grain cannot be perfectly removed by the pickling before the annealing of the hot rolled sheet, and the magnetic properties are deteriorated as state above.
Further, in the annealing of the hot rolled sheet, it is necessary to perfectly precipitate AlN for satisfied ferrite grain growth at a final annealing, and coarsen the precipitated AlN, for which a sufficient soaking time should be carried out in the annealing. If the soaking time is short and the coarsening of AlN particles is insufficient, the grain growth at the final annealing is spoiled by the inhibiting effect of movements of the grain boundaries due to AlN particles.